1. Field of Invention
The present invention relates generally to methods for coupling high speed data components and particularly to a method and apparatus for coupling high speed data components using imbedded PCB AC coupling capacitors.
2. Description of Related Art
Transmitting signals between emitter coupled logic (ECL) circuits that have different bias levels (such as 3.3 and 5 volts, respectively) typically requires alternating current (AC) coupling to isolate direct current (DC) components of the transferred signal. AC coupling can be achieved with a coupling transformer or one or more series capacitors. In either case, the addition of these components can be problematic. Transformers are relatively expensive and large, and at times, difficult to mount to printed circuit boards (PCBs). Separate discrete component capacitors are often smaller than transformers, but not small enough to avoid the routing and layout problems inherent in the typically densely populated PCBs. AC coupling capacitors must be designed such that the impedance of the capacitor is sufficiently low at the frequency of interest to pass the desired signal through substantially unattenuated. These problems are further exacerbated when the signals sought to be transmitted using the FCL circuitry are high bandwidth signals such as those in the order of 100 MHz or more, such as NRZ 8B/10B encoded serial Fibre-channel data, whose lowest switching frequency is about 106 MHz, with the resulting edge rate harmonics around 2 GHz. At such frequencies, current flow across the capacitor plates can result in substantial impedance distortions, reflecting waves, and other undesirable non-linearities.